ACIDS, BASES, AND SALTS 177 



are called strong electrolytes. Substances which give poorly con- 

 ducting solutions are called weak electrolytes. 



4. Vapor Pressures; Boiling-Points; Freezing-Points. 



We have seen (pp. 117-9) that equal numbers of molecules of 

 different solutes dissolved in equal weights of water normally 

 depress the vapor pressure, raise the boiling-point, and lower 

 the freezing-point by constant amounts. Thus, one molecular 

 weight of sugar (342 g.) or of glycerine (92 g.) dissolved in 1000 

 g. of water will raise the boiling-point from 100 to 100.52, and 

 will lower the freezing-point from to 1.86. But this is 

 uniformly true only of non-conducting solutions, in other words, 

 solutions of substances which are not acids, bases, or salts. Gram- 

 molecular weights of substances of these three classes, dissolved 

 in 1000 g. of water, raise the boiling-point more than 0.52 degrees 

 and lower the freezing-point by more than 1.86 degrees. We 

 say they give abnormal elevations of the boiling-point and ab- 

 normal lowerings of the freezing-point. Vapor pressure depressions 

 in such solutions are also abnormal. 



Thus, a solution of 58.46 g. of sodium chloride in 1000 g. of water 

 boils at 100.97, and freezes at -3.42. The elevation in the 

 boiling-point of the water is 0.97 instead of 0.52. The depres- 

 sion of the freezing-point is 3.42 instead of 1.86. The effect 

 in each case is nearly twice as great as the normal one. In the 

 same way, a gram-molecular weight of potassium chloride dis- 

 solved in 1000 g. of water at 20 depresses the vapor pressure 

 by 0.554 millimeters, while a gram-molecular weight of mannite 

 (a normal sugar which gives a non-conducting solution) in the 

 same amount of water lowers the vapor pressure by only 0.313 mm. 

 Again the effect is nearly twice the normal. 



The only conclusion we can draw from these results is that 

 nearly twice the normal numbers of solute molecules are present 

 in such solutions. In other words, not only do we have two inde- 

 pendent sets of properties exhibited by the constituent radicals 



